Because of their high efficiency, class D amplifiers are particularly useful for portable and compact audio applications, such as in automotive applications.
A typical class D amplifier system 10, shown in FIG. 1, features a class-D audio amplifier 12 that transforms an incoming audio signal 14, which can be analog or digital, into an amplified pulse-coded representation of that incoming signal, hereafter referred to as the amplifier output 16. A reconstruction filter 22 converts this amplifier output 16 into a corresponding analog audio signal 20, which can then be used to drive a speaker 18. This process is carried out by a reconstruction filter 22. An important component of this reconstruction filter 22 is an inductor.
Inexpensive and coupled or uncoupled compact planar inductors 24, such as that shown in FIG. 2 can be used in reconstruction filters 22 of class-D amplifier systems 10. A planar inductor 24 typically includes a printed circuit board 26 having concentric traces 28 that function as the windings of the inductor. First and second low-reluctance structures 30, 32 resting on opposing sides of the printed circuit boards function as the core of the inductor 24. Typical low-reluctance structures 30, 32 often comprise but are not limited to a ferromagnetic material, such as powdered iron, or ferrite. In an effort to linearize the change in inductance as a function of frequency, such structures often incorporate air gaps. These air gaps can result in cross-talk caused by magnetic linkage between nearby inductors.